In monitoring the operational state of an electrical apparatus, it is often useful to monitor the power used by the apparatus. This is particularly true of electrical apparatuses that are designed to operate with electrical power supplies of differing voltages, such as apparatuses designed to operate at either 120 v of U.S. commercial power supplies or 240 v of European power commercial supplies. For such apparatuses, it is insufficient to monitor only the current used, since the current might be acceptable at one voltage level whereas the same current would be unacceptably high at another voltage level.
Detecting power, however, is not straightforward, since it involves a multiplication of current by voltage. Multipliers in general are complicated to fabricate, and to fabricate accurately. Moreover, the very concept of multiplication becomes unclear when it is considered that most commercial power supplies provide an alternating AC current.